Bottom Line:
These results correlate with bacterial virulence in the Caenorhabditis elegans host model as well as human clinical data.Our results demonstrate that the D. melanogaster model is potentially useful for studying S. aureus pathogenicity.Different MRSA strains demonstrated diverse virulence in the D. melanogaster model, which may be the result of differing expression of bacterial virulence factors in vivo.

Background: Staphylococcus aureus strains with distinct genetic backgrounds have shown different virulence in animal models as well as associations with different clinical outcomes, such as causing infection in the hospital or the community. With S. aureus strains carrying diverse genetic backgrounds that have been demonstrated by gene typing and genomic sequences, it is difficult to compare these strains using mammalian models. Invertebrate host models provide a useful alternative approach for studying bacterial pathogenesis in mammals since they have conserved innate immune systems of biological defense. Here, we employed Drosophila melanogaster as a host model for studying the virulence of S. aureus strains.

Results: Community-associated methicillin-resistant S. aureus (CA-MRSA) strains USA300, USA400 and CMRSA2 were more virulent than a hospital-associated (HA)-MRSA strain (CMRSA6) and a colonization strain (M92) in the D. melanogaster model. These results correlate with bacterial virulence in the Caenorhabditis elegans host model as well as human clinical data. Moreover, MRSA killing activities in the D. melanogaster model are associated with bacterial replication within the flies. Different MRSA strains induced similar host responses in D. melanogaster, but demonstrated differential expression of common bacterial virulence factors, which may account for the different killing activities in the model. In addition, hemolysin α, an important virulence factor produced by S. aureus in human infections is postulated to play a role in the fly killing.

Conclusions: Our results demonstrate that the D. melanogaster model is potentially useful for studying S. aureus pathogenicity. Different MRSA strains demonstrated diverse virulence in the D. melanogaster model, which may be the result of differing expression of bacterial virulence factors in vivo.

Mentions:
We tested both feeding and pricking methods to compare the virulence of clinical MRSA strains in the fly model. Feeding experiments did not show significant differences among these strains in terms of the killing activities (data not shown). However, pricking experiments demonstrated that different clinical MRSA strains had distinct killing activities. Flies injected with plain BHI broth were included as a negative control, for which no flies were killed during the whole period of the experiment. USA300, USA400 and CMRSA2, previously shown to have a greater propensity to cause clinically invasive human infection [6], demonstrated high killing activities, with 51.4%, 60.3% and 72.8% of flies dead at 36 hours, and 83.5%, 84.9% and 97.7% of flies dead at 72 hours, respectively. No significant differences were observed between these strains (p>0.05) (Figure 1A). However, CMRSA6 showed significantly lower killing activity (p<0.05), whereby only 15.3% of flies died at 36 hours and 71.8% at 72 hours. Moreover, the colonization strain M92 showed significantly lower killing activity compared with CMRSA6 (p<0.05). To further confirm the differential fly killing activities described above, two additional clinical isolates from each clonal group with similar genetic backgrounds were tested. It was noted that all isolates belonging to the same clonal group demonstrated similar killing activities (p>0.05) (Figure 1B-E). However, all the members of each clonal group from USA300, USA400 and CMRSA2 showed significant differences to all the members of CMRSA6 group (all p<0.05), but no significant differences were observed between all the strains of each clonal groups from USA300, USA400 and CMRSA2 (all p>0.05). Taken together, these results confirmed that USA300, USA400, and CMRSA2 strains were highly virulent in the fly model, while CMRSA6 and M92 were considered to be of lower virulence.

Mentions:
We tested both feeding and pricking methods to compare the virulence of clinical MRSA strains in the fly model. Feeding experiments did not show significant differences among these strains in terms of the killing activities (data not shown). However, pricking experiments demonstrated that different clinical MRSA strains had distinct killing activities. Flies injected with plain BHI broth were included as a negative control, for which no flies were killed during the whole period of the experiment. USA300, USA400 and CMRSA2, previously shown to have a greater propensity to cause clinically invasive human infection [6], demonstrated high killing activities, with 51.4%, 60.3% and 72.8% of flies dead at 36 hours, and 83.5%, 84.9% and 97.7% of flies dead at 72 hours, respectively. No significant differences were observed between these strains (p>0.05) (Figure 1A). However, CMRSA6 showed significantly lower killing activity (p<0.05), whereby only 15.3% of flies died at 36 hours and 71.8% at 72 hours. Moreover, the colonization strain M92 showed significantly lower killing activity compared with CMRSA6 (p<0.05). To further confirm the differential fly killing activities described above, two additional clinical isolates from each clonal group with similar genetic backgrounds were tested. It was noted that all isolates belonging to the same clonal group demonstrated similar killing activities (p>0.05) (Figure 1B-E). However, all the members of each clonal group from USA300, USA400 and CMRSA2 showed significant differences to all the members of CMRSA6 group (all p<0.05), but no significant differences were observed between all the strains of each clonal groups from USA300, USA400 and CMRSA2 (all p>0.05). Taken together, these results confirmed that USA300, USA400, and CMRSA2 strains were highly virulent in the fly model, while CMRSA6 and M92 were considered to be of lower virulence.

Bottom Line:
These results correlate with bacterial virulence in the Caenorhabditis elegans host model as well as human clinical data.Our results demonstrate that the D. melanogaster model is potentially useful for studying S. aureus pathogenicity.Different MRSA strains demonstrated diverse virulence in the D. melanogaster model, which may be the result of differing expression of bacterial virulence factors in vivo.

Background: Staphylococcus aureus strains with distinct genetic backgrounds have shown different virulence in animal models as well as associations with different clinical outcomes, such as causing infection in the hospital or the community. With S. aureus strains carrying diverse genetic backgrounds that have been demonstrated by gene typing and genomic sequences, it is difficult to compare these strains using mammalian models. Invertebrate host models provide a useful alternative approach for studying bacterial pathogenesis in mammals since they have conserved innate immune systems of biological defense. Here, we employed Drosophila melanogaster as a host model for studying the virulence of S. aureus strains.

Results: Community-associated methicillin-resistant S. aureus (CA-MRSA) strains USA300, USA400 and CMRSA2 were more virulent than a hospital-associated (HA)-MRSA strain (CMRSA6) and a colonization strain (M92) in the D. melanogaster model. These results correlate with bacterial virulence in the Caenorhabditis elegans host model as well as human clinical data. Moreover, MRSA killing activities in the D. melanogaster model are associated with bacterial replication within the flies. Different MRSA strains induced similar host responses in D. melanogaster, but demonstrated differential expression of common bacterial virulence factors, which may account for the different killing activities in the model. In addition, hemolysin α, an important virulence factor produced by S. aureus in human infections is postulated to play a role in the fly killing.

Conclusions: Our results demonstrate that the D. melanogaster model is potentially useful for studying S. aureus pathogenicity. Different MRSA strains demonstrated diverse virulence in the D. melanogaster model, which may be the result of differing expression of bacterial virulence factors in vivo.